JED - September 2016 - 49

COHERENCE

SOLID-STATE LASERS
After years of development, most modern IRCM systems
now use solid-state lasers, but it has been a relatively long
maturation cycle. Back in 1984, I had the task of choosing
an infrared laser to use in a lab where we were doing IRCM
experiments funded by the Air Force. Up until that point, our
lab had always used lamps to generate the jamming signals for
experiments. At a time when IRCM solid-state lasers were in
their infancy, I soon discovered that I only had two options.
I could get a Deuterium Fluoride chemical laser (It's just as
noxious as it sounds, and it came with a recovery pack to make
sure the active ingredients wouldn't render me unconscious on
the lab floor.), or a Helium-Xenon gas laser that was close to
5 feet long and took up most of my optical table. I chose the
gas laser, which had a maximum output of 30 milliwatts, just
enough for lab experiments but a thousand times less than
what is needed in an operational IRCM system. I used to "tune"
it for maximum output by observing the hue of the gas in the
laser cavity, which was an adjustable mixture of 95% Helium
and 5% Xenon.
The other important feature of the laser that, like temporal coherence, is arguably both a blessing and a curse is the
laser's spatial coherence, which provides a very narrow beam
compared to the diffuse beams of infrared lamps. Just as with
the lamps, the output beam can be shaped by external optics,
but there is a severe system penalty for deviating from the
laser beam's narrowness. In fact, the strength of the beam is
inversely proportional to the square of the beam's angular size.
In other words, a beam of 2 degrees would have its strength
(the precise term for strength in this case is "radiant intensity," which is power-per-solid-angle, or watts-per-steradian),
decreased by 4 times if the beam angle just doubles to 4 degrees. This relationship between beam size and strength became an underpinning principle to IRCM system designers who
used lasers, because it meant that for a fixed amount of power,
there would be a big trade-off between beam strength and
beam width.
To preserve the beam strength, designers attempted to keep
the beam width small, but that meant it had to be precisely
pointed at the missile, unlike in the case of the lamp systems,
which were either loosely pointed or not pointed at all. Precisely directing the laser beam required: 1) having a cue from
a missile warning system that the missile was approaching,
and 2) a device that could take the cue and point the laser at
the infrared missile. Because the missile is moving at twice the
speed of sound as it approaches the aircraft, the pointing device also had to be able to track the missile very accurately in
order to keep the laser pointed at the missile's seeker head during its speedy trajectory. This gave birth to the term "pointer/
tracker" to describe the subsystem that effectively wields the
"light saber" at the missile in what are now commonly known
as directable IRCM (DIRCM) systems.

The Journal of Electronic Defense | September 2016

Due to its temporal coherence, a laser emits at a specific
spectral line, or "color." This is different from a lamp, which
emits over a broad spectrum. Because infrared missiles operate in several different spectral bands, an infrared jammer
must be able to simultaneously emit in several parts of the
infrared spectrum. This means that using laser technology
in the jammer requires having more than one laser source
to get the equivalent spectral coverage provided by a single lamp. Furthermore, generating even a single laser line
where missiles operate is more difficult than in other parts
of the electro-optical spectrum where semiconductors and
gases naturally emit. Thus, generating the multiple laser
colors, or lines, in the infrared became a focus of development work in industry, at DARPA and at other Government
labs in the 1980s.
Different approaches emerged for getting laser light into
the missile bands and into IRCM systems. An early system
used a CO2 gas laser that was optically converted to a missile band by exciting a Silver Gallium Selenide (AgGaSe2)
crystal. There were also solid-state laser approaches that
used direct semiconductor output or a combination of electrical and optical pumping of other electro-optically active
crystals, like AgGaSe2, to generate the infrared light. On a

practical note, often these electro-optically active crystals
were in short supply because they had to be grown with just
the right "recipe" over a long period of time in temperaturecontrolled furnaces.

49



JED - September 2016

Table of Contents for the Digital Edition of JED - September 2016

The View From Here
Conferences Calendar
Courses Calendar
From the President
The Monitor
World Report
Nothing to See (Or Hear) Here – Concealing Tactical Communications
Technology Survey: Manpack COMINT and DF Systems
The Heat is On
EW 101
AOC News
Index of Advertisers
JED Quick Look
JED - September 2016 - cover1
JED - September 2016 - cover2
JED - September 2016 - 3
JED - September 2016 - 4
JED - September 2016 - 5
JED - September 2016 - The View From Here
JED - September 2016 - 7
JED - September 2016 - insert1
JED - September 2016 - insert2
JED - September 2016 - Conferences Calendar
JED - September 2016 - 9
JED - September 2016 - Courses Calendar
JED - September 2016 - 11
JED - September 2016 - From the President
JED - September 2016 - 13
JED - September 2016 - 14
JED - September 2016 - The Monitor
JED - September 2016 - 16
JED - September 2016 - 17
JED - September 2016 - 18
JED - September 2016 - 19
JED - September 2016 - 20
JED - September 2016 - 21
JED - September 2016 - 22
JED - September 2016 - 23
JED - September 2016 - 24
JED - September 2016 - 25
JED - September 2016 - 26
JED - September 2016 - 27
JED - September 2016 - World Report
JED - September 2016 - 29
JED - September 2016 - 30
JED - September 2016 - 31
JED - September 2016 - 32
JED - September 2016 - 33
JED - September 2016 - Nothing to See (Or Hear) Here – Concealing Tactical Communications
JED - September 2016 - 35
JED - September 2016 - 36
JED - September 2016 - 37
JED - September 2016 - 38
JED - September 2016 - 39
JED - September 2016 - 40
JED - September 2016 - 41
JED - September 2016 - 42
JED - September 2016 - Technology Survey: Manpack COMINT and DF Systems
JED - September 2016 - 44
JED - September 2016 - 45
JED - September 2016 - 46
JED - September 2016 - 47
JED - September 2016 - The Heat is On
JED - September 2016 - 49
JED - September 2016 - 50
JED - September 2016 - 51
JED - September 2016 - EW 101
JED - September 2016 - 53
JED - September 2016 - AOC News
JED - September 2016 - 55
JED - September 2016 - 56
JED - September 2016 - Index of Advertisers
JED - September 2016 - JED Quick Look
JED - September 2016 - cover3
JED - September 2016 - cover4
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